This invention relates to amplifiers for driving a load through an alternating-current path with a constant-amplitude, varying duty cycle signal. An example of such an amplifier is a driver for a horizontal output transistor of a television deflection system.
Television display systems conventionally scan a lighted raster area on the face of a picture tube by recurrently scanning an electron beam vertically at a relatively low rate and horizontally at a relatively high rate. Because of the high horizontal scanning rate, it is important to reduce the amount of energy converted to heat during each horizontal deflection cycle in order to reduce the total power consumption. To this end, the horizontal deflection is commonly accomplished by the use of electronic switches, which may be turned on and off at the high deflection rates and which in principle consume no power in either switching state.
When the horizontal deflection switching device is a bipolar transistor, the base must be driven with sufficient current during the conductive or ON time of the transistor to allow the collector current to be controlled by the collector circuit rather than by the base current. In this saturated operating condition, excess or surplus charge carriers are stored in the base region of the transistor. As is known, it is necessary to sweep the base region clear of stored charge carriers before control of the collector current by the base can be reasserted. That is, in order to switch the transistor into the nonconducting or OFF state, a reverse voltage must be applied to the base-emitter junction for an interval sufficient for removal of the excess charge carriers. Only after the excess carriers are removed does the collector current cease. The time required to switch the transistor into the nonconducting condition after application of a reverse bias to the base-emitter junction depends upon the geometry of the transistor and upon the number of excess charge carriers. For a given geometry and base current, the number of excess carriers decreases with increasing collector current, and consequently the time required to switch the transistor into the nonconducting state decreases as the collector current increases.
The images to be displayed are encoded with timing information in the form of composite video. The picture information content is separated from the synchronizing signals in the television receiver and modulates an electron beam which illuminates the raster area. The synchronizing signals are applied to the deflection circuits for controlling the deflection timing in a manner to allow the coded images to be displayed.
In current television practice, the horizontal output stage is called upon to produce direct voltage for energizing various portions of the television receiver as well as producing deflection current. The loads may be variable and of substantial magnitude, as for example the picture tube ultor, which may require 60 watts or more and which varies with picture content. The variations in the load presented to the horizontal output stage causes the peak collector current of the horizontal output transistor to vary from time to time. Thus, the switch-off time of the transistor may vary, thereby causing variations in picture timing and apparent distortion of the images displayed on the raster.
A feedback loop arranged to control the deflection timing circuits in such a manner as to compensate for delay variations in the horizontal output stage is described in U.S. Patent Application Ser. No. 948,775, filed Oct. 5, 1978 in the name of Balaban et al. In the Balaban arrangement, a phase-control loop varies a delay circuit interposed between the source of synchronizing pulses and the horizontal output transistor driver circuits. The phase-control loop compares the horizontal retrace pulses with the synchronizing signals and reduces the phase error therebetween.
Since the recurrent deflection interval as defined by the interval between horizontal synchronizing pulses is constant, variation in the drive signal timing to control the turn-off time of the horizontal output transistor may cause a duty-cycle variation of the horizontal drive signal. If the driver stage is AC (alternating-current) coupled to the base of the horizontal output transistor as for example by a transformer as described in U.S. Pat. No. 3,302,033 issued to Goodrich, the duty cycle variation of the constant-amplitude drive signal may result in unwanted amplitude variations of the horizontal drive signal at the base. For example, an increase in horizontal output transistor collector current resulting from an increase in beam current results in a decrease in the delay between the application of a negative-going portion of the drive signal to the base of the horizontal output transistor and the resulting turn-off time and retrace pulse. If uncorrected, the retrace pulse will be advanced in time relative to the encoded video information, and the encoded information will be relatively delayed. As viewed on the raster, the picture information will be displaced to the right from the desired position. A phase-correction loop will delay the application of the turn-off portion of the base drive in a compensating manner. This, however, may increase the duration of that portion of the horizontal drive signal tending to keep the horizontal output transistor conductive. Since the interpulse period is fixed, this decreases the OFF portion of the drive signal (that part of the drive signal tending to turn the output transistor OFF), resulting in a duty cycle change. The drive signal when AC coupled must have equal positive and negative-going portions relative to the average value of the signal at the base of the horizontal output transistor, with the result that the increase in the duration of the ON-portion of the drive signal decreases its amplitude and increases the amplitude of the OFF portion. The decreased amplitude of the ON portion of the horizontal output transistor drive signal reduces the base current during this interval. The reduced base current further reduces the excess charge carriers and further enhances the decrease in storage time. Thus, the combination of duty cycle modulation and AC coupling undesirably reduces the effective gain of the phase-control loop.
A self-regulating arrangement is described in the article "A New Horizontal Output Deflection Circuit" by Peter Wessel, which appeared in the IEEE Transactions on Broadcast and Television Receivers, Vol. BTR-18, August, 1972. In the Wessel arrangement, a horizontal output transistor is AC coupled to a driver by a transformer. Such driver transformers are commonly used to store energy during the ON-time of the driver transistor and to couple the energy to the base of the following horizontal output transistor during the OFF-time of the driver transistor. The drive waveform has a fixed turn-off time and a turn-on time which is variable under the control of a voltage feedback loop. In the Wessel arrangement, the variation of the ON-portion of the drive waveform results in a duty cycle variation which varies the delay between the time the reverse bias voltage is applied to the base of the horizontal output transistor and the resulting retrace pulse. Undesirable distortion in the displayed image results.
It is desirable to maintain the AC coupling between the driver and the horizontal output transistor, and to correct or avoid the timing errors described above resulting from duty cycle variations. It is also desirable to reduce the effective voltage applied to the driver stage as energization potential without incurring dissipative losses. More generally, it is desirable to have an amplifier which when AC-coupled to a load and driving the load with a duty-cycle varied signal produces across the load a constant-amplitude portion relative to the average or direct voltage value.